Research

Some of our recent research highlights include the use of observational insights of the variability of low-level cloudiness in the trades to evaluate numerical weather prediction and climate models. Additionally, we are working towards a better characterization and understanding of the trade-wind cumuli in their role as precipitating shallow systems. We also continue our contributions to international assessments of clouds, aerosol and radiation.

1. Low-level cloudiness in the trades

Within the trades, climate models diverge in their predictionof shortwave cloud radiative effects and climate sensitivity. Using observations at Barbados, our group has studied the major cloud modes in the trades. Despite large variations in the mean state that accompany the seasonal cycle in theNorth Atlantic trades, cloudiness near cloud base, or the lifting condensation level, is found to be relatively constant,being closely coupled to the surface fluxes. Most of the variability in total cloudiness on longer time scales is instead carriedby variations in cloud aloft, such as stratiform layers near the detrainment level of cumulus tops at 825 hPa (Figure 1). This is a marked contrast with many climate models, in which variations in cloudiness are mostly carried by variations in cloud near cloud base. Several climate models also show the opposite seasonality in the stratiform component. This suggests that models vary clouds in ways that are different from nature, which implies that modeled cloud feedbacks may be unrealistic.

Figure 1. The profile of cloud fraction (CF) during the dry season from January through March (left), during the wet season from August through October (middle), and the difference in cloud fraction between the dry and the wet season (right). The Barbados cloud observations are shown in black, the ECMWF short and long integration forecasts in light and dark blue, and a subset of CMIP5 models in other colors. The model data are for a single gridbox just upstream of Barbados. The Figure illustrates the wide variety of cloud fraction profiles present in the models, and the inability of several models to capture the seasonality in the cloud fraction profile. In the observations (BCO), the reduction in cloud fraction near cloud base (950-925 hPa) in the dry season is primarily due to the upward shift of the lifting condensation level. Changes in cloud fraction between the two seasons is overall small, except for an increase in cloud fraction between 850-825hPa, which reflects the more frequent occurrence of stratiform layers near the detrainment level of cumulus tops. These layers can also be seen in the photograph taken from HALO in Figure 4 below. From Nuijens et.al. [3].

Nuijens, L., B. Medeiros, I. Sandu and M. Ahlgrimm, 2014: The behavior of trade-wind cloudiness in observations and models: the major cloud components and their variability, Journal of Advances in Modeling Earth Systems, submitted [draft PDF].

2. Precipitating shallow systems

Low intensity rainfall from shallow convective systems in the trades likely has a non-negligible contribution to global oceanic precipitation (Figure 2). The exact contribution of these systems to total rainfall however is uncertain, given the difficulty of observing them from space using passive microwave sensors and the lack of ground-based measurements of rainfall over the open ocean. Using radars deployed at the Barbados Cloud Observatory, we have evaluated the ability of three well-known satellite products of global oceanic rainfall (GPCP, TMPA and HOAPS) to reproduce the climatological and day-to-day mean rainfall over the Northern Atlantic. This comparison shows that the satellite sensor's overestimation of the area covered by rain, due to their coarse resolution, outweighs their underestimation of the occurrence of light rain, leading to on average larger daily rain-rates in the trades compared to ground-based radar observations. This can give a misleading picture of the general ability of satellite sensors to capture light rain events [4].

If rainfall from shallow clouds is indeed non-negligible, it may be crucial for the energy budget of the trades. Precipitation can help set the equilibrium structure of the trade-wind layer, and helping it adjust to a changing large-scale flow. The role of precipitation and its associated heating and moistening tendency is explored in a new PhD project using idealized Large-Eddy Simulations.

Another question of interest is how the aerosol may regulate rainfall in these shallow systems. Using the Barbados cloud radar data, we have analyzed how the microphysical structure of trade-wind cumuli is altered during Saharan dust events. On dusty days, clouds that do not yet rain have a weaker increase of reflectivity (Z) with height than non-dusty days (Figure 3). This hints at a reduction of droplet sizes with an increase in number of dust particles, which would impact rain formation. However, when conditioning on ambient relative humidity, the dust dependency vanishes. This can be explained from theory and a simple entraining plume model, which shows that the Z-gradient is more sensitive to small changes in relative humidity, which impacts the liquid water content in clouds, than to large changes in droplet number concentration, which is regulated by aerosol [5].

Figure 3. The reflectivity (Z) gradient, from theory (black) and from observations (red/blue), which approximately scales with β2/N, where β is the sub-adiabaticity of clouds, and N the droplet number (aerosol) concentration. Reducing β2/N from 0.69 (non-dusty) to 0.45 (dusty) can be achieved by a ~ 50% increase in N, or instead a ~20% decrease in β, which is already achieved by a ~0.5% absolute decrease in relative humidity. From Lonitz et al. [5]

These findings also emphasize the necessity of observing the humidity structure along with cloud systems. The first set of research flights with HALO (Figure 3), carried out successfully in December 2013 and intersecting the A-Train satellite overpass on every flight leg, has provided such combined measurements. These data have already demonstrated that not just at the BCO, but also over the open ocean across the broader Atlantic, organized convective towers with stratiform outflow near their tops are emerging as one of the key cloud modes in the trades.

Figure 4. A photograph taken during the first HALO-South campaign. Courtesy: Bjorn Stevens.

To assess whether satellite data are sufficiently accurate to be used for understanding the climate system or to help constrain parameterizations in climate models, data from ground-based stations are crucial. Our group's contributions to measurements of cloud and aerosol have been used for such purposes. A comparison of the cloudiness record collected at the BCO and that of MODIS has placed confidence in the seasonality of low-level cloud amount in the North-Atlantic trades, which appears to be relatively invariant around a mean value of 30%, but with a minimum of 20% in late Summer/early Fall [see Brueck et al, 2].

Furthermore, over the past four years twenty cruises on German Research Vessels have provided aerosol optical data over remote oceans under the umbrella of the Marine Aerosol Network (MAN, AERONET). These data have helped to fill gaps in observed global maps of aerosol properties, which in turn have been used to derive a monthly climatology of aerosol radiative and cloud-active properties (Figure 5). Using the present day response to sources, this climatology has been extended back in time to 1850 and into the future and serves as a basis for a fast and more simplified representation of the aerosol in climate models, and thus contributed to the CMIP5 effort organized within the CMO [6].